bug: checking against NaN is always false

[conradsnicta]

The following statement in fastplm.cpp always evaluates to false:

interflex/src/fastplm.cpp

Line 178 in 560d317

if (coef(i) == arma::datum::nan) {

According to the documentation for std::numeric_limits::quiet_NaN()

A NaN never compares equal to itself.

Armadillo uses std::numeric_limits<T>::quiet_NaN() for datum::nan, so the same caveat applies.

To fix this, use std::isnan() instead:
if (std::isnan(coef(i))) {